eruption cloud

Photographs taken by Space Shuttle astronauts about
24 hours after the start of the eruption of Rabaul Caldera in 1994.
The eruption column rose to at least 18 km above sea level where the
volcanic ash and gas were blown west to form a fan-shaped eruption
cloud. A smaller eruption cloud (bottom photograph, lower right) was
blown northward by lower-level winds.
Credit: NASA

A cloud of tephra and gases
that forms downwind of an erupting volcano.
The vertical pillar of tephra and gases rising directly above a vent is
an eruption column.

Eruption clouds are often dark colored – brown to gray – but
they can also be white, very similar to weather clouds. Eruption clouds
may drift for thousands of kilometers downwind and often become increasingly
spread out over a larger area with increasing distance from an erupting
vent (note fan-shaped eruption cloud in photographs at left). Large eruption
clouds can encircle the Earth within days.